This invention relates to seals for blocking the escape of acoustic wave energy, e.g., engine noise, from an enclosure via an opening. In particular, the invention relates to acoustic seals installed in a gap in a vibro-acoustic treatment applied to an enclosure having an opening.
Typical marine engines are noisy, especially when being operated at higher rpm""s while driving a vessel rapidly through the water. This noisy operation is extremely unattractive to occupants of the vessel, as well as to passers-by, and it is highly desirable to reduce this noise without reducing vessel efficiency. Further, regulatory bodies, in their desire to improve the environment, are imposing emission standards on marine vessels. These standards not only regulate the contents of the emissions but also apply to the noise level of the emission. It is therefore highly desirable to provide a marine engine that is noise reduction efficient without detracting from the vessel operating efficiently.
More general than the noise reduction is noise control. Noise control requires an understanding of the vibro-acoustic behavior of the article in question with its environment. If boundary conditions permit, approximations can be made by isolating the article from its environment. This cannot be done xe2x80x9csimplyxe2x80x9d for an integrated structure. For example, an outboard marine engine is an integrated structure. To capture correctly the vibro-acoustic behavior of an outboard engine, the engine should be fully assembled, mounted to a boat and in the open water. For example, feedback from the added inertia of the water as the boat travels in the water could produce a narrow-band spectrum different from a steady-state condition. There is also feedback from the components of the engine, for example, the crankshaft and block can produce a phenomenon that does not exist for either part acting alone.
To determine the acoustic xe2x80x9cfingerprintxe2x80x9d for an integrated structure such as an outboard marine engine, a narrow-band analysis must be performed. This will allow identification of tones, i.e., frequency responses, of the interacting components. The components corresponding to these responses can be identified from the frequencies, i.e., based on wavelength and speed of sound. Vibro-acoustic treatments can be designed and or critically placed to attenuate or simply move a tone from one frequency to another. The effectiveness of this effort is based on the precision of the data and the methodology by which the data is acquired.
The precision of the data is a function of the frequencies of the data collected and of the transducer sensitivity. The frequency range of interest is a function of human hearing, i.e., 10 kHz is sufficient. For the present work, data was collected using accelerometers and microphones. Accelerometer data was collected to 5 kHz at 1 Hz bandwidth; microphone data was collected to 10 kHz at 2.5 Hz bandwidth. Acoustic intensity testing and stethoscopic probing both showed agreement that over 80% of the vibro-acoustic energy produced by a particular outboard marine engine was coming from below the interface between the engine""s upper and lower motor covers.
To suppress noise emitted by a motor encased in a motor housing having an opening, a vibro-acoustic treatment can be applied on the inner surfaces of the motor housing. However, acoustic wave energy blocked, i.e., trapped, by the vibro-acoustic treatment will leak out any gaps in the treatment, e.g., through an opening in the housing where the treatment is not applied. Thus there is a need for a structure which can reduce the amount of acoustic wave energy escaping via the opening.
The present invention is directed to an improved engine having means for controlling and reducing the noise escaping the motor housing via an opening. This is accomplished by installing at least one acoustic seal that extends across a substantial portion of the opening. In accordance with the preferred embodiment, each seal comprises an attachment portion which is adhered to the housing wall adjacent the opening and a flexible, yet self-supporting, membrane made of material which substantially blocks the transmission of acoustic wave energy therethrough. The membrane preferably comprises rubber.
In accordance with the most preferred embodiment, the acoustic seals are used in conjunction with a shroud applied to the inner surface of the housing. The shroud comprises a blanket of material that both damps vibrations and blocks/absorbs acoustic wave energy. Such material will be hereinafter referred to as a xe2x80x9cvibro-acoustic treatmentxe2x80x9d. The presence of an opening in the motor housing produces a gap in the noise-suppressing shroud, which gap is partly filled by acoustic seals in accordance with the preferred embodiment. The vibro-acoustic treatment comprises an acoustic barrier laminated to an open-cell foam core that absorbs acoustic energy. In accordance with the preferred embodiment, the motor housing comprises upper and lower motor covers, the opening for the steering arm being formed in the lower motor cover. Respective vibro-acoustic treatments are applied to the upper and lower motor covers. The vibro-acoustic treatment inside the upper motor cover is designed to shift acoustic energy in the frequencies between 1000 and 3000 Hz to frequencies below 1000 Hz and above 3000 Hz. In contrast, the lower motor cover can be considered the primary receiver of the structure-borne noise and vibration. To best attenuate this energy, the vibro-acoustic treatment for the lower motor cover is designed to attenuate across a wide frequency range (e.g., 0 to 4,000 hertz), but was optimized for the frequencies under 1000 Hz. The vibro-acoustic treatment on the upper motor cover was not designed to attenuate frequencies below 1,000 hertz because to do so would require additional mass inside the upper motor cover, which additional mass would negatively impact overall engine performance, e.g., by interfering with the intake of air.
Although the preferred embodiment is disclosed in the context of an outboard marine engine, persons skilled in the art will readily appreciate that the means for noise suppression could also be installed inside a housing encasing the powerhead of an inboard marine engine or any other type of engine. (The terms xe2x80x9cpowerheadxe2x80x9d and xe2x80x9cmotorxe2x80x9d will be used interchangeably throughout the written description and the claims.)